Thoughts, please. I've been shooting a Nikon D810 for several years, generally pleased with it but it's slow to focus, a problem now that I shoot more birds. Does the 850 focus faster? Or is it better in other ways? Other suggestions? I can't switch to mirrorless: my Mac computer is so old (OSX 10.13.6) it can't be upgraded and, according to Nikon techies, the mirrorless software can't accommodate the older software. Thanks.